Trivia

Although Poliwag and Poliwhirl were able to learn Amnesia in Generation I, they are not able to learn the move in any other generation, and thus they and their evolved forms can only have the move in Generation II if they are traded forward.

Despite several Pokédex entries, especially in the anime, said that Psyduck can't remember while using mysterious power, it could not learn this move until Generation IV, except via a Pokémon Stadium Nintendo event.